Densely-Graded Aggregate Base

Transcription

1 Transportation Kentucky Transportation Center Research Report University of Kentucky Year 1959 Densely-Graded Aggregate Base William B. Drake Kentucky Highway Materials Research Laboratory This paper is posted at UKnowledge. researchreports/1328

2 COMMONWEALTH OF KENTUCKY DEPARTMENT OF HIGHWAYS FRANKFORT June 12., 1959 Ward J. Oates COMMII>SIONER OF HIGHWAYS ADDRESS REPLY TO DEPARTMENT OF HIGHWAYS MATERIALS RESEARCH LABORATORY 132 GRAHAM AVENUE LEXINGTON 29, KENTUCKY B MEMO TO: SUBJECT: A. 0. Neiser Assistant State Highway Engineer Density of Dense-Graded Aggregate Base; Ref., Your Memo 5c>15-59 Density requirements for crushed graded aggregate base courses are necessary to obtain the desired performance, i.e. to develop shearing strength and to eliminate subsequent consolidation. The load transmission of a particular base material is thereby indirectly dependent upon density, Listed below are some of the factors that affect the density of densegraded aggregate base constructed from crushed limestone. 1. Specific Gravity and ;Porosity of Solid Stone. The specific grav ity usea-is termed "bulk specific gravity (SSD basis) " by ASTM and is the saturated surface dry weight divided by the bulk volume of the stone particles. Limestones that will meet Kentucky stone base requirements will normally have less than 1% of water absorption which would be less than 3% of the solid volume of the stone. By Kentucky density requirements, it is more difficult to obtain density on the porous stones with water absorption of over 1o/o. However, I do not believe that this has been a major factor in our recent compaction problems. 2.. Stone Gradation. A long gradation range and a m1n1mum of 5% passing the No. 200 sieve ( 1- inch top size} is most essential. The fines are needed to fill the voids and act as a binder in the aggregate. 3. Moisture Content at Time of Compaction. The aggregate gradation is compactable if it is at or near optimum moisture content. This moisture content, for best results, appears to be slightly higher than that necessary for saturation at 85o/o solid volume of the stone. It is best, therefore, to mix and place the

3 A. 0. Neiser June 12, 1959 stone a little on the wet side; and, it has been no ted that upon drying, the stone will shrink somewhat and obtain a greater density. It is possible that the water necessary for compaction, particularly that adhering to the fine particles will hold the stone apart. This effect is pre dominate in the bulking of wet sands. The Department has placed dense graded aggregate with and without moisture control additive (calcium chloride). The Research Division has reported densities on projects of each type. Fayette County RS , Southland Drive in Lexington, was the first dense-graded limestone base placed in Kentucky. One-half ofone percent calcium chloride was used and the DGA was placed over a single course of waterbound macadam. The following year, the Phil-Pine Grove road, Casey County F 50:0, was constructed using DGA without moisture control additives. On this project, the DGA was placed over existing TBM. Average densities on the Fayette County project were 8. 6 lbs. per cu. ft. greater than on the Casey County project. The Fayette County DGA, while placed under the same specifications, did have a greater percentage of fines. The percentage of voids on the Fayette County project averaged 9. 9% while the Casey County section averaged 15. lo/o. Both of the above projects were wetted and mixed in place. It has been reported by A. M. Johnson, J. A. Knight, and F. 0. Slate that maximum density granular soils and base materials could be obtained with less cornpactive effort when moisture control chemicals are used (HRB Bibliograp-hy 24). 4. Cornpactive Effort and De '?I._Layers. These items include the concentration of the cornpactive load, type of cornpaction, and the number of repetitions of the load. In a paper presented at the 1959 Kentucky Highway Conference and published in the April issue of_!l.rnerica Highways, Mr. John J. Laing of the Bureau of Public Roads emphasized the basic principles of compaction. Mr. Laing's paper, "Cornpaction of Asphaltic Concrete", also discussed compaction of granular base courses and equipment requirements. He mentioned that base course layers were normally placed in thicknesses of 3 to 5 in., but that maximum thicknesses of 8 to 10 in. have been reported. In Fig. 2, which deals with the pressure distribution through a cornpactable layer, he shows that 0. 9 of the tire contact pressure is distributed to a depth of 4 in., while the pres sure is reduced to 0. 5 at a depth of 8 in. At 13 in. the effective pressure is only 0. 3 of the contact pressure at the surface. Mr. Laing concludes from his discussion, quote: "It would seem preferable to develop an "end result" specification where density or other finished characteristics in addition to profile and crown tolerances would be specified".

4 A. 0. Neiser June 12, 1959 Compactive effort is dependent upon three factors: ( 1) roller contact pressure, (2) depth of layer, and (3) number of repetitions or passes or roller. These three items are independent and can be varied within reasonable ranges to obtain desired densities. The depth of layer can not be in creased appreciably without increasing the contact pressure of the roller. 5. Sub grade Support or Reactance. It was mentioned in item 3 above that the DGA on Fayette RS was placed over a course of WBM, while the Casey County project was placed over TBM. The WBM undoubtedly provided a better reactance for compaction than the TBM. Some of the density variables can be attributed to this reactance; but, of course, other factors were involved there also. A dense, dry subgrade should provide ample reactance for plantc,mixed DGA. Wet o.r soft subgrades can affect the densities obtained. 6. Method of Test. The method of test for density (rubber balloon) of DGA is primarily a soils or fine aggregate test. At the time the turnpike insulation course was being constructed, it was decided that more than one method of density test would be investigated. The general consultant's mate rials engineer made density comparison tests at various locations using calibrated sand, paraffin coated chunks, and the rubber balloon. On the average it was found that the density results were higher when tested with the rubber balloon than by the sand method, but the paraffin coated samples that were used for reference varied from location to location. The average difference in density was in the order of 2 lbs. per cu. ft. It was decided, however, to use the rubber balloon in calculating the volume of the hole because of the convenience it afforded and because the variations in density test compari sons were only minor. The turnpike insulation was placed in 6-in. compacted layers. Various compaction equipment was used for the different contracts. On June 8, 1959, I visited the Hardin County section of Interstate I 65. Plant-mix DGA base was being compacted in a 6-in. layer. Pneumatic rollers and Jackson-type vibrators were being employed..some water was being added during the shaping and compaction operation. Some difficulty was encountered in obtaining the 85o/o solid density before a satisfactory compaction procedure was decided upon. It was also noted by the project engineer that density could not be obtained if the subgrade was wet and did not provide the necessary reactance. Density tests were being conducted after the base had cured to about 3o/o moisture. It appeared that the compaction procedures (initial rolling with pneumatic followed by Jackson vibro tampers) were producing the required densities.

5 A. 0. Ne iser 4 - June 12, 1959 I am of the opinion that the density reguirements of 85% solid volume as calculated from the bulk specif ic gravityof th.:o stone is realistic and practical. It is' somewhat severe f<>r certain stones but should go a long way toward the elimination of rutting in flexible base courses. I believe that consideration should be given to a modified sand density test such as Tennessee uses, particularly if the top size of DGA is increased over the pre sent 1 in. The rubber balloonctype test would not be suitable for the coarser aggregates. I prefer an end result specification for DGA and believe that the choice of thicknesses of layer and type of roller should be left to th.e contractor. Density can be affected by either item. WBD :dl Encs. Respectfully submitted, W. B. Drake Associate Director of Research